Conventionally, a rotation angle sensing device for detecting a rotation angle of a rotating body has been used for various purposes. One such known rotation angle sensing device is provided with a magnet that is fixed so as to rotate integrally with a rotating body, and a magnetic sensor element for detecting a change in intensity of a magnetic field in association with rotation of the magnet. In such a rotation angle sensing device, the magnetic sensor element outputs a signal indicating a relative positional relationship between the rotating body and the magnetic sensor element.
A known conventional rotation angle sensing device, as shown in FIG. 31, includes a magnet 200 that is formed like a disk and is supported by and fixed to a shaft S (rotary shaft) so as to allow a first surface 201 and a second surface 202 of the magnet 200 to be orthogonal to the shaft S (rotary shaft), and a magnetic sensor element (Hall element) 300 that is arranged immediately beneath an outer circumference of the second surface 202 of the magnet 200, and it a circumferential direction centering on the shaft S (rotary shaft) (see Patent Literature 1).
In the rotation angle sensing device, because shaft wobble occurs and causes the shaft S (rotary shaft) to minutely move, the magnet 200 supported by and fixed to the shaft S (rotary shaft) minutely moves in the radial direction. In the meantime, the magnetic sensor element (Hall element) 300 is arranged to as to measure magnetic flux density in a direction that is parallel with the shaft S (rotary shaft) at a corner portion(s) of the outer circumference of the magnetic 200. Consequently, a measured value for magnetic flux density that is measured by the magnetic sensor element (Hall element) 300 varies greatly in association with the minute movement of the magnet 200, and an error in measurement of a rotation angle is accentuated.
Conventionally, as shown in FIG. 32, a rotation angle sensing device is proposed that is provided with two magnets 210 that are supported by and fixed to the shaft S (rotary shaft) across a void in the axial direction of the shaft S, and two magnetic sensor elements (Hall elements) 310 that are arranged between rotary shaft directions of the two magnets 210, and between an outer circumferential end of the shaft S and that of the magnets 210 (see Patent Literature 2).